Fuel cells have been studied as a likely candidate for an efficient low emission power source. This is because fuel cells offer the prospect of silent, modular technology for energy generation with few moving parts and little or no toxic emissions. In operational fuel cells, a liquid or gaseous fuel on the anode side and a liquid or gaseous oxidant on the cathode side are separated by an ion conducting electrolyte that prevents these reactants from directly contacting each other. The fuel cell properties and subsequent use in special niche markets, is determined by whether the electrolyte is liquid or solid.
In the case of electric vehicles and mobile, man-portable systems, where lightweight and high power densities by weight and volume fuel cells are needed, development efforts have centered on fuel cells using a solid, ion conducting Polymer Electrolyte Membrane (PEM). A distinctive feature of this type of fuel cell has been that a porous polymer electrolyte membrane (PEM) from about 50-200 microns in thickness in the form of a proton conducting membrane (or film) simultaneously acts as the separator as well as the medium for ionic transport. Use of PEM instead of electrolytes made of aqueous solutions, molten salts, or solid oxides has liberated this particular fuel cell technology from assembly, storage and maintenance problems commonly encountered in using corrosive and/or high temperature liquid electrolytes. Fuel cells using PEMs are expected to have a long shelf life on open circuit stand, and can start-up and operate at temperatures below 100.degree. C. since PEM do not require high temperatures to achieve good ionic conductivity. Operating voltages of PEM fuel cells have been largely determined by the PEM ionic conductivity that is a strong function of its water retention, while current efficiencies have been inversely related to permeation of the anode fuel through the membrane. It would be of obvious interest to increase both the water retention and ionic conductivity, and decrease the permeation of fuel through these PEMs and in general increase the overall performance of fuel cells using these membranes.
Currently, hydrated membranes based on salts of perfluorosulfonic acid (perfluorosulfonates) developed by E. I. Dupont of Wilmington, Del. under the trade name Nafion, and a lower molecular weight analog developed by Dow Chemical Company of Midland, Mich. are currently among the more popular PEMs used in fuel cells. Their chemical structures are basically a sulfonated analog of Teflon with pendant side chains terminating in SO.sub.3 H groups. Although these perfluorinated membranes have excellent chemical stability and relatively high protonic conductivity, they are expensive, and available only in certain ranges of thicknesses and specific ionic conductivities. It would be desirable to increase the water retention and ionic conductivities of these membranes. Another fundamental drawback to using PEM in fuel cells has arisen when these membranes have been used in fuel cells using other fuels such as methanol. Since these polymers can be considerably permeable to other fuels such as methanol, the fuel from the anode side can diffuse though the membrane to become the fuel spontaneously oxidized at the cathode side. This direct chemical short reduces the overall cell potential and leads to fuel wastage. If diffusion of either the oxidant or fuel through the PEM could be reduced, fuel cells using fuels other than hydrogen could become a practical reality.
In addition, the surface of the PEM is usually coated with a catalyst layer in order to catalyze the anode or cathode reaction. The interface between the applied catalyst layer and the PEM is important because in order for the fuel cell to operate efficiently, ions must travel through this layer and into the PEM. Improved contact between the catalyst layer and the PEM is important for proper fuel cell performance.
Various methods of improving ionically conducting membranes have been proposed.
In U.S. Pat. No. 5,128,014, Banerjee discusses irradiation of membranes with high energy radiation such as beta rays, gamma rays or x-rays. This procedure is costly, and suffers from safety problems associated with the radioactive species and the maintenance costs. In U.S. Pat. No. 4,439,292, Klotz and Fitzky discuss treating membranes with a corona discharge where an electric discharge or spark is passed from one electrode to the other. The #292 treatment involves applying 1-20 kV across the membrane in either air or carbon dioxide at atmospheric pressure. The #292 patent teaching is also limited to membranes containing carboxylic acids or have been first modified to contain carboxylic anode groups before treatment.